Influence of the structure of cetyltrimethylammonium bromide based microemulsions on base hydrolysis of carboxylic acid esters

Article abstract of DOI:10.1023/B:RUGC.0000045863.38879.c8

The structure of water/cetyltrimethylammonium bromide/n-butanol/hexane microemulsions was studied by conductometry, viscometry, and Fourier transform ¹H NMR spectroscopy with pulse magnetic field gradient. The regions of phase inversion from in

Full text of DOI:10.1023/B:RUGC.0000045863.38879.c8

Russian Journal of General Chemistry, Vol. 74, No. 7, 2004, pp. 1051 1056. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 7, 2004,
pp. 1140 1145.
Original Russian Text Copyright
2004 by Zuev, Mirgorodskaya, Kudryavtseva, Idiyatullin, Khamidullin.
Influence of the Structure of Cetyltrimethylammonium Bromide
Based Microemulsions on Base Hydrolysis
of Carboxylic Acid Esters
Yu. F. Zuev, A. B. Mirgorodskaya, L. A. Kudryavtseva,
B. Z. Idiyatullin, and R. N. Khamidullin
Kazan Institute of Biochemistry and Biophysics, Kazan Scientific Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
Received July 15, 2002
Abstract The structure of water/cetyltrimethylammonium bromide/n-butanol/hexane microemulsions was
1
studied by conductometry, viscometry, and Fourier transform H NMR spectroscopy with pulse magnetic field
gradient. The regions of phase inversion from inverse micelles in hexane through a bicontinual structure to a
dispersion of normal micelles in water were determined. The influence of the structure of the microemulsions
on the rate of hydrolysis of carboxylic acid p-nitrophenyl esters was analyzed. The hydrolysis rate constants
considerably increase in going from inverse to normal microemulsions.
A specific feature of microemulsions containing a
large amount of cationic surfactants is the possibility
of varying the component ratio in the system in a
wide range. In so doing, the structure and properties
of microemulsions change essentially, and the phase
inversion and rearrangement of the interface take
place. Transitions are possible from inverse micro-
emulsions (water microdroplets dispersed in nonpolar
solvents and stabilized by a layer of surfactants and
cosurfactants) to bicontinual systems in which the
surfactant molecules form a three-dimensional network
and water and oil form two continuous phases and
then to normal microemulsions (oil microdroplets in
water). In such transitions, the system remains macro-
homogeneous and optically transparent; it preserves
high solubilizing power toward both polar and non-
polar substances [1 3].
To reveal the influence exerted by the structure of
the reaction medium on the rate of nucleophilic sub-
stitutions, we studied base hydrolysis of carboxylic
acid p-nitrophenyl esters in water/cetyltrimethylam-
monium bromide/n-butanol/n-hexane microemulsions
under the conditions of phase transitions initiated by
variation of the water : oil ratio. The structure of the
microemulsions was studied by a set of physical
methods allowing determination of the ranges of
phase inversion in the system.
One of the best ways to monitor structural transi-
tions in microemulsions is measuring their electrical
conductivity [6 10]. The possible sources of free
charges in the aqueous phase are charged head groups
of ionic surfactants, their counterions, or ionic addi-
tives. The conductivity of the oil phase is lower than
that in water by several orders of magnitude. For
water/oil microemulsions, the main charge carriers are
inverse micelles dispersed in the organic phase. For a
number of water/oil microemulsions, small variations
of parameters (e.g., variations of component concen-
trations or temperature) cause a dramatic (by up to
4 5 orders of magnitude) increase in the electrical
conductivity (percolation) caused by increased effi-
ciency of intermicellar exchange, primarily with water
and dissolved ions, on short agglutination in the
course of clusterization [11 13]. Another possible way
of realization of percolation transitions is formation of
bicontinual structures with long water channels along
The latte fact is extremely important when a micro-
emulsion is used as a medium for chemical processes,
ensuring efficient contact of reactants that differ in the
solubility in water and in organic solvents. Localiza-
tion of the reactants in the system and their microen-
vironment depend on the type and properties of micro-
emulsions. By varying the ratio of the dispersed phase
and dispersion medium, it is possible to vary the site
of localization of the reaction event and reaction part-
ners and thus to control the rate and mechanism of
chemical processes [4, 5].
1070-3632/04/7407-1051 2004 MAIK Nauka/Interperiodica

1052
ZUEV et al.
of the dispersed phase, the microdroplets form a clus-
ter, creating a route for charge transfer between the
electrodes; as a result, the electrical conductivity
sharply increases [8, 11, 13]. Figure 1 shows that the
conductivity of the microemulsion increases by four
orders of magnitude in the range of low water concen-
trations, which is typical of manifestation of percola-
tion in inverse micelle dispersions. As the water con-
centration is increased further, the electrical conduc-
tivity (Fig. 1) shows characteristic features apparently
reflecting changes in the microemulsion structure.
However, the conductivity data alone do not furnish
unambiguous structural information. In this work, to
elucidate the structure of the reaction medium, we
300
200
100
0
100
10
1
0.1
0.01
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Volume fraction of water
Fig. 1. Electrical conductivity of the microemulsion
based on cetyltrimethylammonium bromide as a function
of the water content.
1
also used viscometry and Fourier transform H NMR
spectroscopy with pulse magnetic field gradient. At
the same time, the conductometric method is extreme-
ly illustrative and allows the more labor-consuming
methods to be used more purposefully.
which the charge can be transferred [6]. Depending on
the microemulsion morphology, both mechanisms of
the electrical conductivity may be operative; this is
primarily determined by the nature of the surfactant
(structure of the polar head group, site of the charge
localization, length of the hydrocarbon radical), pres-
ence and type of a cosurfactant, temperature, etc.
Furthermore, a bicontinual structure can form beyond
the range of percolation transition [7].
To determine the diffusion coefficients D of micro-
emulsion components, we used the strongest signals
1
in the H NMR spectra. The diffusion coefficients of
water and butanol were determined, respectively, from
the signals of water protons (4.80 ppm) and -CH2
protons of butanol (3.76 ppm). For cetyltrimethyl-
ammonium bromide, we chose the proton signal of its
head CH3 groups, shifted to 3.43 ppm owing to inter-
action with the positively charged nitrogen atom. The
signals of the (CH2)n protons of hexane overlap with
the related signals of cetyltrimethylammonium bro-
mide and butanol. At a high content of hexane, the
diffusion decays of the proton signals of the (CH2)n
groups of different components (1.47 ppm) are well
resolved. At the minimal content of hexane ( w 0.77),
the diffusion coefficient of hexane coincides with that
of cetyltrimethylammonium bromide (DCTAB). The
diffusion coefficients obtained are listed in the table.
Figure 1 shows the electrical conductivity data for
the microemulsions based on cetyltrimethylammoni-
um bromide. The conductivity of water/oil micro-
emulsions is determined by the diffusion of micro-
droplets bearing an noncompensated fluctuation elec-
tric charge [8, 9]:
kBT
0
(1)
=
,
r3
2
11
1
where 8.85 10 F m is the dielectric constant;
0
, dielectric permittivity of the organic medium; kB,
Boltzmann constant; T, absolute temperature; , vis-
cosity of the organic medium; r, hydrodynamic radius
of micelles; and , volume fraction of microdroplets
in the system.
Using the modified Stokes Einstein relationship
[Eq. (2)] taking into account direct collisions between
microdroplets, we determined from the diffusion coef-
ficients the mean hydrodynamic radii r of microdrop-
For the most dilute water/oil microemulsion with
lets in two microemulsions characterized by
and 0.77.
0.01
w
the volume fraction of water in the system
0.01,
w
using Eq. (1), we estimated the size of microdroplets,
which appeared to be 5.3 nm. This is quite reasonable,
as the length of the cetyltrimethylammonium bromide
molecule is 2.5 nm [14] and microdroplets also con-
tain water and butanol molecules. In calculations, we
assumed that butanol is incorporated in micelles in-
completely; as and , we took the corresponding
values for hexane.
kBT
(2)
DCTAB
=
(1 2 ).
6
r
As
we took the viscosity of the pure solvent
(water or hexane) forming the dispersion medium in
each specific case. We found that, for inverse micelles
(
0.01), r 4.9 nm, which is well consistent with the
valwue obtained conductometrically. The mean hydro-
dynamic radius of direct micelles ( 0.77) appeared
As the concentration or size of microdroplets in-
creases, their density in the oil medium and the prob-
ability of collisions grow. At a definite concentration
w
to be 3.2 nm, which is close to the previously ob-
tained result for the similar microemulsion based on
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 7 2004

INFLUENCE OF THE STRUCTURE OF CETYLTRIMETHYLAMMONIUM BROMIDE
1053
cetylpyridinium bromide [15]. Thus, in the extreme
points of the concentration range studied, the micro-
emulsion structure can be described as aqueous and
organic microdroplets stabilized by a layer of the
surfactant and dispersed in the medium of the oppo-
site polarity.
Diffusion coefficients of microemulsion components
Volume DCTAB
Dbutanol
1010,
Dwater
1010,
Dhexane
1010,
fraction
of water
1010,
1
1
1
1
m2 s
m2 s
m2 s
m2 s
0.035
0.19
0.25
0.28
0.42
0.53
0.77
0.95
1.08
10.0
8.5
2.4
6.4
9.4
10.2
12.2
14.4
14.4
32.0
25.0
20.0
19.0
10.5
The range of transition of the solvents (water or
hexane) from the dispersed state to the bulk phase can
be clearly traced in Fig. 2. The D/D0 plots (Fig. 2a),
where D0 is the self-diffusion coefficient of the corre-
sponding pure solvent, reflect the set of factors affect-
ing the diffusion of the solvents. On the one hand, as
the micelles grow in size, their diffusion mobility and
hence the mobility of the encapsulated solvent should
decrease. On the other hand, the probability of ex-
change between micelles with their contents should
simultaneously increase, thus promoting the diffusion
of the encapsulated solvent [13]. As the dispersed
phase concentratin increases, the hindrance to motion
of the dispersion medium grows. At a high concentra-
tion of the dispersed phase, when the diffusion mobil-
ity of molecules of both solvents could be expected to
decrease, formation of a bicontinual structure, relative-
ly open for the diffusion transport of both water and
hexane, is probable. Furthermore, the partial mutual
solubility of the two liquids (though low) should be
taken into account. From the viewpoint of variation of
1.48
1.01
0.70
0.31
8.7
4.6
6.3
4.0
0.66
0.41
sion in the range
0.01 0.2 corresponds to disper-
sion of inverse micewlles in hexane; in the range of
0.2 0.25, it transforms into a bicontinual structurwe
which is preserved up to
0.4 0.5; and at higher
water content, a normal mwicroemulsion is formed.
The microemulsions in hand were studied as a
medium for base hydrolysis of carboxylic acid p-nitro-
phenyl esters:
RC(O)OC6H4NO2 + 2OH
R = CH3, C11H23.
The observed rate constants of the hydrolysis (kobs
in the microemulsions linearly depend on the concen-
tration of hydroxide ions. Some examples are shown
in Fig. 3.
RC(O)O + C6H4NO2,
the diffusion coefficients, the critical region is
)
w
0.25 for water and
0.42 0.5 for hexane.
w
As compared to diffusion data, changes in the
widths of the resonance lines (Fig. 2b) are more pro-
nounced. As the water fraction is increased to
0.2 0.25, the size of inverse micelles increases owinwg
to the growth of their aqueous cores, and the water in
these cores only slightly differs in the characteristics
The plot of kobs of hydrolysis vs. volume fraction
of water at 0.01 M alkali content in the microemulsion
(Fig. 4) consists of three pronounced regions whose
boundaries are close to the points of transition from
from the bulk water. At
0.2 0.25, water, appar-
ently, gets into the networkwof narrow channels of the
bicontinual structure; the surface of these channels
strongly affects averaging of the dipole interaction of
water protons, which is manifested in broadening of
the line at 4.8 ppm. In the same concentration range,
the resonance line of the (CH2)n protons broadens
jumpwise (Fig. 2b), but the amplitude of this effect is
relatively low because of fairly large width of this line
inverse microemulsions to bicontinual structures (
w
0.25) and then to normal microemulsions. Variation of
kobs caused by structural rearrangements is a resultant
of several oppositely acting effects. On the one hand,
the alkali concentration in the aqueous phase changes:
in going from the microemulsion with
with
by a factor of 5. This makes an appreciable positive
contribution to the hydrolysis rate constant in the case
of microemulsions with a low water content. On the
other hand, in normal microemulsions the transfer of
the substrate encapsulated in oil microdroplets of
radius 3.2 nm to the interface on which the reaction
with OH takes place occurs more readily than in the
systems where oil forms a bulk phase. Also, with an
increase in the water content in the system, the polar-
ity of the medium in the site of the reaction event can
increase, which is a favorable factor for SN2 reactions
such as the base hydrolysis studied in this work.
0.75 to that
0.14, the content of hydroxide wion increases
w
even at
0.01 (reference value) and redistribution
of contribwutions to this line from cetyltrimethyl-
ammonium, hexane, and butanol with changing
composition.
In the same concentration range, the microemulsion
viscosity dramatically grows (Fig. 2b), which also
suggests formation of a bicontinual structure stabilized
with cetyltrimethylammonium bromide and butanol
throughout the volume of the system. Combined anal-
ysis of the plots in Fig. 2 suggests that the microemul-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 7 2004

1054
ZUEV et al.
(a)
1
1
kobs, s
1
0.08
0.06
0.04
1
0.1
2
2
0.01
3
0.02
(b)
4
10
8
0
2
4
6
8
10
cNaOH 103, M
1
Fig. 3. Observed rate constant of hydrolysis of car-
boxylic acid esters in various microemulsions as a func-
tion of alkali concentration: (1) p-nitrophenyl acetate,
6
0.74; (2) p-nitrophenyl acetate,
0.35; (3) p-nitro-
4
2
w
w
2
phenyl acetate,
0.14; and (4) p-nitrophenyl laurate,
w
0.74.
w
When the alkali concentration in the aqueous phase is
made constant, it becomes possible to reveal the effect
of the microenvironment of the reactants on kobs
(Fig. 4). The laurate appears to be more sensitive to
structural rearrangements as compared to its short-
chain homolog: In going from a microemulsion with
the lowest water content to that with the highest water
content, the hydrolysis rate constant increases by
1 order of magnitude in the case of p-nitrophenyl
acetate and by 2 orders of magnitude in the case of
p-nitrophenyl laurate. The chosen esters differ in their
hydrophobicity: The distribution constant between oil
and water is 10.8 for the acetate and 205.4 for the
larate [4]. Presumably, the laurate is localized in the
oil phase, and the acetate, closer to the surface layer.
The microenvironment of the acetate changes less
strongly, compared to the laurate, with structural
transformations of the microemulsions, which is re-
flected in the hydrolysis rate constants. Also, it should
be taken into account that significant changes in the
viscosity, accompanying the structural transforma-
tions, can also contribute to the hydrolysis rate in the
form of the diffusion constituent; its role is especially
significant in bicontinual microemulsions [1].
0
(c)
12
11
10
9
8
7
6
5
4
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Volume fraction of water
Fig. 2. (a) Diffusion coefficients of (1) water and (2)
hexane (relative to those in the respective pure solvents),
(b) width of the resonance lines of (1) water protons and
(2) (CH ) protons (total of signals of hexane, cetyltri-
methylammonium bromide, and butanol) relative to the
2 n
width at
0.01, and (c) microemulsion viscosity, as
Thus, combined use of conductometry, viscometry,
and NMR spectroscopy allowed evaluation of a num-
w
functions of the volume fraction of water.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 7 2004

INFLUENCE OF THE STRUCTURE OF CETYLTRIMETHYLAMMONIUM BROMIDE
1055
1
1
(a)
(b)
k
obs, s
kobs, s
0.08
0.015
0.06
0.04
0.02
0
2
0.010
2
1
0.005
0
1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Volume fraction of water
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Volume fraction of water
Fig. 4. Observed rate constant of hydrolysis of (a) p-nitrophenyl acetate and (b) p-nitrophenyl laurate as a function of the
volume fraction of water in microemulsions at constant concentration of alkali (0.01 M) in the (1) microemulsion and
(2) aqueous phase.
ber of characteristic parameters of microemulsions
based on cetyltrimethylammonium bromide at widely
varied hexane/water ratio and determination of the
range of transition from inverse microemulsions to bi-
continual systems and then to normal microemulsions.
Variation of the rate of the base hydrolysis of carbox-
ylic acid p-nitrophenyl esters is associated with struc-
tural rearrangements of the microemulsions: The reac-
tion rate considerably increases in going from inverse
to normal microemulsions, which is determined by the
site of reactant localization in the system.
The molecular structure of microemulsions was
studied by Fourier transform H NMR spectroscopy
1
with pulse magnetic field gradient [16, 17]. Diffusion
measurements were performed with a modified Tesla
BS-587A NMR specrometer at a proton resonance fre-
quency of 80 MHz. The spectrometer was equipped
with a unit generating a pulse magnetic field gradient
1
of up to 50 G cm .
For kinetic studies, we used carboxylic acid
p-nitrophenyl esters purified by common procedures.
The reaction kinetics was studied spectrophotometri-
cally in freshly prepared microemulsions in tempera-
ture-controlled cells of a Specord UV-Vis device.
The reaction progress was monitored by variation of
the optical density of the solutions at 400 nm (forma-
tion of p-nitrophenolate anion). The initial concentra-
EXPERIMENTAL
Phase inversion in microemulsions based on cetyl-
trimethylammonium bromide (Merck) was studied at
20 C. As the medium we chose a system containing
0.942 g of cetyltrimethylammonium bromide and
0.942 g of butanol (molar ratio of surfactant to cosur-
factant 1 : 5), with which the phase separation on
successive variation of the water : hexane ratio within
wide limits could be avoided. In NMR experiments,
we used D2O (Aldrich) instead of double-distilled
water.
5
tion of the substrate was 5 10 M; the reaction was
performed to >90% conversion.
The observed pseudo-first-order rate constants
(kobs) were determined by least-squares treatment of
the dependence log(D
D ) = 0.434kobs + const,
where D and D are the solution optical densities at
moment and after reaction completion, respectively.
The electrical conductivity was measured with a
Radelkis OK-102/1 conductometer (Hungary) at fre-
quencies of 80 Hz and 3 kHz in a temperature-con-
trolled plane-parallel cell. The viscosity was measured
with a capillary Ostwald viscometer.
ACKNOWLEDGMENTS
The study was financially supported by the Russian
Foundation for Basic Research (project no. 02-03-
32923).
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1056
ZUEV et al.
J. Phys. Chem., 1989, vol. 93, no. 1, p. 314.
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